Shock absorbers which force fluid through a restricted orifice to convert the kinetic energy of a moving part into an increase in the thermal energy of the fluid are commonly used on machines. The smoothest deceleration of the moving part is obtained by absorbers which offer a constant resistive force to the motion over the total length of the deceleration.
One class of such devices employ a piston connected to the machine part and movable within a cylinder having a closed end. A series of exponentially spaced holes are formed along the length of the cylinder wall and the cylinder is supported within a housing filled with fluid. As the piston is forced into the cylinder by motion of the machine part, the fluid is forced through the holes and the kinetic energy of the part is converted into thermal energy of the fluid. As the piston moves down the cylinder it successively closes off the holes so that the force imposed on the load is maintained relatively constant resulting in a linear deceleration of the moving part.
The force imposed on the part is a function of the configuration of the fluid orifices, and linear decelerators of this class have been designed wherein the orifice configuration may be varied to accommodate the device for use with parts having varying weights and kinetic energy. One of the most common approaches is to provide grooves in a tubular sleeve fitting over the cylinder. The grooves in the sleeve cooperate with the holes in the cylinder to define the fluid orifices. The angular position of the sleeve on the cylinder may be adjusted to vary the orifice configuration and, thus, the resistance provided to the load. Representative examples of the so-called "groove-on-hole" shock absorbers are disclosed in commonly assigned U.S. Pat. Nos. 4,059,175; 4,298,101; and 4,321,987, as well as the disclosures in non-related U.S. Pat. No. 3,425,522 to Gryglas and U.S. Pat. No. 3,693,767 to Johnson. While devices having this orifice configuration have generally proved satisfactory in operation, there is a tendency for leakage to occur through the seals between the cylinder and the sleeve. The groove design provides a potentially shorter lower resistance path for oil to seep from the holes in the cylinder through the mutually facing walls of the cylinder and the sleeve. This leakage disturbs the optimum device characteristics since the fluid flow is not constrained to flow solely through the preselected orifice configuration.
As exemplified by U.S. Pat. No. 3,510,117 to Scholin et al and U.S. Pat. No. 3,840,097 to Holly, the prior art does disclose the use of holes instead of grooves in the outer sleeve. The Holly patent employs a plurality of triangularly disposed openings in the inner cylinder and the sleeve contains an axially extending row of openings therethrough. The cylinder is rotated radially to selectively vary the number of overlapping openings to adjust the energy absorbing capacity of the device. This construction, like the groove-on-hole design, is subject to leakage due to the multitude of potential leakage paths provided by the nonused openings in the triangular arrangement in the cylinder. In the Scholin et al patent, a relatively complex construction is provided for axially moving the outer sleeve to a finite number of positions relative to the holes in the cylinder. As a consequence, there is a limited number of preset orifice configurations thereby limiting the ability of the device to be individually tailored to the precise characteristics of the moving part, and the device is relatively expensive to manufacture.